1. Field of the Invention
The invention relates to a device for metering a reducing agent into an exhaust system. Such devices are used particularly for reducing pollutants in exhaust gases.
2. Description of the Prior Art
In internal combustion engines, in particular diesel engines, because of the increasingly stringent exhaust gas regulations coming in the next few years, among other things the proportion of nitrogen oxides in the exhaust gas will have to be reduced. For reducing the proportion of nitrogen oxide in the exhaust gas, selective catalytic reduction (SCR) is for instance performed, in which the nitrogen oxides are reduced with the aid of reducing agents to nitrogen and water. As reducing agents, an aqueous urea solution, for instance, is employed. Typically, the pollutant NOx is reduced to N2 and H2O with the aid of liquid reducing agents.
The reducing agent is typically stored in a tank and pumped via a line from the tank to a metering module, with which the metering module is injected, for instance into the exhaust tube. One example of such a device is shown in European Patent Disclosure EP 0 982 884 A2.
The metering module typically has a metering valve, which is received by an adapter. This adapter with the metering valve can be mounted on or screwed onto the exhaust tube.
One problem in such metering modules with a metering valve and an adapter, however, is that a switching operation of the metering valve causes a great deal of noise. A switching operation (for instance, opening or closing the metering valve) leads to structure-borne sound in the structure, in particular the adapter, surrounding the valve. This structure-borne sound is transmitted from the outer faces of the adapter to the surrounding air and emitted as airborne sound.
From the field of automotive technology, various possibilities for acoustic insulation are already known in the prior art. Besides insulation mats and insulation foams of woven fabrics, and mineral fibers or polymer foams, metal sound absorbers are sometimes also used. One example of such metal sound absorbers is disclosed in German Patent Disclosure DE 103 45 575 B3. In it, a device for acoustic and thermal shielding is proposed that has a platelike element with a layer of a metal cloth with warp filaments and weft filaments. The warp and weft filaments are joined together at their contact points by sintering, in such a way that the metal cloth has pores.
One problem in using such special metal insulating materials, however, is that such materials are comparatively expensive compared to conventional materials, and that conventional machining and production processes that are usual in automotive technology cannot be employed in all cases.